The side windows slide in plastic tracks due to the curve of the side panels. I used 3/16" lexan. Coated to help prevent scratches. Definitely not cheap. It worked out pretty well but was a real pain to plan and get set up. The plastic tracks aren't very deep and you have to account for expansion. My boat is stored outside (but under a roof) and I think I had to allow for about 1/8" vertical expansion/contraction. (+30C summer to -40C winter). The mahogany trim will ultimately hold them if they come out of the tracks. I'll put some pictures up to help show what I did. Quite happy with the outcome but many times I thought that I should have made some flat sections in the cabin side panels and purchased windows.
Hope this helps. Of all the different jobs on the boat, the windows were the ones that caused the most headache. Once decisions were made on how I was going to go about it, it wasn't too bad.
Blue tape covers gluing area
Glued solid wood to the inside of the cabin wall, approximately one inch away from the edge of the opening.
Probably about 3/4" thick (width of the track). You may notice that there is no upright to the frame forward of the helm seat. I eliminated it to allow for making the windows longer which also makes for a bigger opening when they are opened. Seems minor but does make for a more useable open window. I felt that there was a lot of strength added by the solid wood glued to the inside of the window frame. Drilled 1/2" drain hole at each end to allow rainwater to get out. Don't even see them. If I were to do it again I would make the inside trim higher than the outside trim along the bottom to not allow water to build up if the drains were plugged. Happened once and got water inside during a bad rainstorm
Shows the curvature of the cabin side
Glued 3/8" solid mahogany to the outside of the cabin wall around the window opening, flush with the cutout in the plywood
Finished product shows the inner trim glued to the wood installed on the inside of the window opening. The top section is attached by screws to allow for removal of the windows/tracks. There ends up being a 1" channel that the tracks sit in all the way around. I added tracks to the upright sections of the opening as well which really stiffened the windows up when they were closed.